The present invention relates generally to a measuring strip, and more particularly to a strip that may be used to measure offsets in rigid steel conduit, electrical metallic tubing, galvanized rigid conduit, rigid aluminum conduit and the like.
When installing conduit to house runs of conductors, the installer inevitably will need to make bends in the conduit run in order to navigate around obstacles. Bends of various shapes are usually needed, such as right angle or 90-degree bends, offsets and saddles. Most, if not all, of these bends will be made on the job as part of the installation procedure and are, therefore, known as field bends.
Before these field bends are actually made, it is essential that the installer do some careful planning. As each bend in the run adds to the friction caused when pulling the conductors, the NEC allows for only a total of 360 degrees of bends between pull points. As more bends are made, more pull points are required. This accordingly will affect the required labor to install the conductors, and ultimately affects the schedules and profits of the job. Therefore, it is good practice for the installer to walk the entire route before any conduit is installed, especially when dealing with longer runs with difficult conductor installation.
The schedules and profits of the job are further affected by the labor and material costs involved in the installation of the conduit, and, as such, accurate measurements arc crucial. While installing electrical conduit for example, it will be necessary to calculate the correct length for a specific run. Because most installations are typically not straight runs, these bends must be made to navigate around obstructions to ensure a precise fit. When two opposing bends that are equal in the degree of their angle are used to clear an obstacle and maintain a course parallel to the original course, it is called an offset bend. In other words, and assuming both bends have the same angle measure, an offset bend is used when an obstruction requires a change in the conduits' plane.
Before making an offset bend, the installer must choose the most appropriate angle for the offset. Although this angle may be primarily chosen with respect to the offset depth, shallow bends will make for easier conductor pulling, while steeper bends will conserve space. Additionally, the installer must also consider that the conduit will shrink due to this detour. This shrinkage or take-up may possibly be ignored if working away from the obstruction, but certainly must be considered when working into the obstruction.
After the installer determines the offset depth and offset angle necessary to clear the obstacle, he will then typically refer to an offset multiplier table to calculate the required distance between conduit bends to achieve the desired dimensions. An example of such an offset multiplier calculation is illustrated below in Table 1.
TABLE 1Offset MultiplierOffset Depth × Constant Multiplier = Distance between bendsColumn 3Column 1Column 2SHRINK PER INCH OFANGLEMULTIPLIEROFFSET DEPTH10° × 10°6 {fraction (1/16)}″ per inch22½° × 22½°2.6 {fraction (3/16)}″ per inch30° × 30°2¼″ per inch45° × 45°1.4⅜″ per inch60° × 60°1.2½″ per inch
To use this table, the installer locates the measured offset angle on the left or first column, then taking the corresponding multiplier constant of column 2, he multiplies this constant by the measured depth of the offset to determine the distance between the offset bends. For example, if the depth of the offset is 5″and the offset angle is 30°, then, according to the Table, 30° corresponds to a 2 constant so that the distance between bends is 5″×2 or 10″. The table further provides for the corresponding shrinkage due to the detour. In this example, the shrinkage (column 3) is 5″×¼″ per inch or 1 ¼ shrinkage.
Tables such as those illustrated by Table 1 certainly aid the installer that encounters the prescribed offset angles depicted by such tables. However, if the installer encounters an offset angle that is not depicted in his table, he must either approximate the measurement or determine the exact distance between bends through the use of trigonometry. As the multiplier constant in these tables is the cosecant of the angle of offset, a complete chart of Natural Trigonometric Functions illustrates all of the constants for any particular degree of offset. Obviously, such a complete chart would hinder the installer at the job site, so smaller tables illustrating the most common offset angles are more commonplace.
In any event, this procedure, when used properly, is an adequate method for the offset bending of conduit runs, but it is often time consuming to refer to tables and make calculations as well as prone to error due to careless approximations. Whenever a mistake is made installing conduit it costs money both in the form of labor and materials. An invention that eliminates this error would be most beneficial to the art because it would eliminate the waste and added expense caused by such errors.
In light of the preceding problems, there has been an effort in the art to develop quick guides for installer use in bending conduit. One such effort, for example, has been developed to incorporate an offset calculator for 45° angles onto a standard tape measure strip. The Trubend™ 45° Offset Calculating Tape Measure attempts to accomplish this goal. Basically, the strip has a standard scale at the top of the rule by which the height of the obstruction is measured. The corresponding measurement on the bottom of the rule is used to measure the spacing between the 45° bends.
However, this method fails to incorporate easily readable pre-marked indicia that illustrate the correct distance between bends of known offset depths and angles. While it does have an offset multiplier table on the back side of the strip for other angles, the installer still must find the correct multiplier constant, make a calculation, and then re-measure for the correct distance.
Other attempts to reduce human error when determining the lengths of piping required for given angles are shown in U.S. Pat. Nos. 2,389,369, 4,696,110, and 4,811,489. All three of these patents have indicia for standard measurement with some form of an additional table or indicia for determining lengths of pipes. Specifically U.S. Pat. Nos. 4,696,110 and 4,811,489 have disclosed tables for determining the take off(or shrinkage) of pipes when using elbow and 45° angle fittings. However, these inventions fail to satisfy the niche required for precise determination of the distance between conduit tubing bends for all of the most commonly used offset angles quickly and precisely.
In view of the aforementioned needs and the shortcomings of the prior art, it is, therefore, a general object of the present invention to provide a measuring strip that will provide typical measuring indicia with integrated indicia for precise measurement of the distance between bends required to allow a specified offset depth and offset angle for electrical conduit installation.
It is another object of the present invention to eliminate the need for additional separate reference tables for determining offset distances.
It is yet another object of the present invention to reduce error and waste created from incorrect approximations and calculations.
Still another object of the present invention is to save time required to install conduit by eliminating steps required to determine the distances between offset bends.
Another object of the present invention is to provide a measuring strip that will provide typical measuring indicia with integrated indicia for the precise measurement of the shrinkage of conduit with respect to specified offset depths and angles.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.